U.S. patent number 4,002,912 [Application Number 05/645,169] was granted by the patent office on 1977-01-11 for electrostatic lens to focus an ion beam to uniform density.
This patent grant is currently assigned to The United States of America as represented by the United States Energy. Invention is credited to Cleland H. Johnson.
United States Patent |
4,002,912 |
Johnson |
January 11, 1977 |
Electrostatic lens to focus an ion beam to uniform density
Abstract
A focusing lens for an ion beam having a gaussian or similar
density profile is provided. The lens is constructed to provide an
inner zero electrostatic field, and an outer electrostatic field
such that ions entering this outer field are deflected by an amount
that is a function of their distance from the edge of the inner
field. The result is a beam that focuses to a uniform density in a
manner analogous to that of an optical ring lens. In one
embodiment, a conically-shaped network of fine wires is enclosed
within a cylindrical anode. The wire net together with the anode
produces a voltage field that re-directs the outer particles of the
beam while the axial particles pass undeflected through a zero
field inside the wire net. The result is a focused beam having a
uniform intensity over a given target area and at a given distance
from the lens.
Inventors: |
Johnson; Cleland H. (Oak Ridge,
TN) |
Assignee: |
The United States of America as
represented by the United States Energy (Washington,
DC)
|
Family
ID: |
24587896 |
Appl.
No.: |
05/645,169 |
Filed: |
December 30, 1975 |
Current U.S.
Class: |
250/396R; 850/1;
313/361.1; 976/DIG.433; 315/506 |
Current CPC
Class: |
G21K
1/087 (20130101); H01J 37/12 (20130101) |
Current International
Class: |
G21K
1/087 (20060101); G21K 1/00 (20060101); H01J
37/12 (20060101); H01J 37/10 (20060101); H01J
037/12 () |
Field of
Search: |
;250/396,397,306,307
;328/228,229 ;313/356,361 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: LaRoche; Eugene R.
Attorney, Agent or Firm: Carlson; Dean E. Zachry; David S.
Deckelman; Louis M.
Claims
What is claimed is:
1. An electrostatic focusing lens for focusing a point source ion
beam of gaussian cross sectional distribution to a uniform cross
sectional distribution at a given distance from the lens,
comprising a grounded inner semi-transparent grid which is
axi-symmetric with said ion beam, said grid having a minimum axial
aperture that matches a central cone portion of said beam, said
grid being a double conical grid including a plurality of equally
spaced metallic wires with a high melting point and a low expansion
coefficient and a grid ring encompassing said grid wires and
defining said axial aperture and provided with a desired radius
with said grid wires being threaded therethrough to thus form said
double conical grid, said lens further including a first grid
support ring, a second support ring, a first pair of spacer rods
mounted between and to said first and second rings for providing a
desired spacing therebetween, a third spring support ring, a second
pair of spacer rods mounted between and to said second ring and to
said third ring, and a plurality of springs equal in number to said
grid wires, said grid wires being attached at first respective ends
thereof to the inner edge of said first support ring with the other
respective ends of said grid wires extending through said second
support ring and fastened to first respective ends of said springs
and the other respective ends of said springs being affixed to the
inner edge of said third spring support ring, and an insulated
outer anode encompassing and spaced from said double conical grid
within said desired spacing between said first and second support
rings, said first, second and third support rings as well as said
grid being at ground potential, said anode adapted to be connected
to a voltage source, said anode shaped such that said voltage
applied thereto provides an electrostatic field between said anode
and said grid, said grid, anode and support rings adapted to be
mounted within an evacuated housing, whereby a portion of the ions
from said ion beam that pass through the inside of said grid are
undeflected by the zero field that exists within said grid, and the
remaining portion of the ions from said ion beam that are outside
of said central cone defined by said grid are deflected by said
electrostatic field toward the axis of said grid an amount
proportional to their minimum radial distance from said central
cone such that the deflected and undeflected portions of said ion
beam merge to produce said uniform distribution at said given
distance from said lens.
2. The lens set forth in claim 1, wherein said grid wires are
molybdenum.
3. The lens set forth in claim 1, wherein said anode in the central
portion thereof has an inner radius three times the radius of said
grid ring with the respective inner end portions of said anode
flaring outwardly.
4. The lens set forth in claim 3, wherein said central portion of
said anode constitutes about half of the length thereof and is
provided with a constant radius in said half, and wherein the
respective outer extremities of said respective inner flared-out
portions of said anode have a respective inner radium about 4 times
the radius of said grid ring.
5. The lens set forth in claim 1, wherein the number of said grid
wires and attached springs is 24.
6. The lens set forth in claim 5, wherein said radius of said grid
ring is equal to 0.33 cm, and the length of said anode is about 8
cm.
7. The lens set forth in claim 6, wherein the diameter of each of
said grid wires is 0.005 cm.
Description
BACKGROUND OF THE INVENTION
This invention was made in the course of, or under, a contract with
the Energy Research and Development Administration.
Ion-induced radiation damage studies are often used to simulate
neutron radiation damage such as described in the application of
Everett E. Bloom et al., Ser. No. 596,546(70), filed July 16, 1975,
and having a common assignee with the present application. These
studies are among the most important of present charged-particle
accelerator applications.
A common requirement of many of the irradiation studies concerns
the fact that specimens are generally examined over a very small
area, and often several specimens are irradiated simultaneously. It
is therefore convenient to work with a beam having a very uniform
beam density profile. Instead of having a uniform density, however,
the typical beam from an ion accelerator has a more gaussian
density profile; i.e., intense along the axis and trailing off
gradually to zero intensity at the beam boundaries.
To achieve uniform impingment density on the target (specimen), the
ions can be focused to a small spot and the beam rastered over the
entire target array. Unfortunately, such a technique introduces an
undesirable time dependence. An allowed procedure, but an
inefficient one, is to place the target out so far along the axis
as to accept only the central part of the divergent beam.
Accordingly, a need exists to provide a special focusing lens
having as its primary object to concentrate the flux of a gaussian
or nearly gaussian ion beam to a nearly uniform distribution onto a
target. The present invention was conceived to meet this need in a
manner to be described hereinbelow.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide means to focus
a beam of ions that emanate with axial symmetry and gaussian or
similar divergence from a small point source to a uniform intensity
over a given target area.
The above object is accomplished by the present invention by
providing a two-field electrostatic lens as follows: a first, field
region is created and which is a field-free central region (ions
incident at a radius r<r.sub.o) and a second field region is
created and is tailored such that ions incident at a radius
r>r.sub.o are diverted toward the central axis by an amount that
is proportional to their distance from the edge of the zero field,
resulting in a beam that focuses to a uniform density at a given
distance from the lens.
This invention is realized herein as a conical network of fine
wires enclosed within a cylindrical anode. The wire net together
with the anode produces a voltage field therebetween that
re-directs the boundary particles of an input ion beam while the
central particles of the input ion beam pass undeflected through
the zero field region inside the wire net. For a gaussian or
similar beam, the result is a focused beam having a uniform
intensity over a given target area.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of an analogous optical ring lens for
focusing a light beam;
FIG. 2 is a schematic illustration of a light beam ray distribution
from a point source utilizing the lens of FIG. 1;
FIG. 3 is a schematic view of the electrostatic ion focusing lens
of the present invention;
FIG. 4 is an isometric structural view of the electrostatic ion
focusing lens of the present invention; and
FIG. 5 is a schematic illustration of one use of the lens of the
present invention for radiation damage studies.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The focusing lens of the present invention can be understood by
first referring briefly to what would be its optical equivalent, as
shown in FIG. 1. I call this lens a ring lens for reasons that will
be apparent. Its inner surfaces are parallel planes and its outer
radii of curvature meet the planes tangentially at the radius
r.sub.o. Thus, it is really two different lenses depending on
whether a ray is incident on the lens at r<r.sub.o, or at
r>r.sub.o.
The unusual optical lens of FIG. 1, could it actually be made,
would produce a light beam ray distribution as illustrated in FIG.
2. Beam rays from a point source A having a radial distribution
that is gaussian or similar that strike the plane surfaces of the
ring lens are undeflected and thus constitute an inner undeflected
cone (rays within A-B and A-C). The rays incident on the ring lens
at r>r.sub.o focus to an image ring at the distance Q and then
diverge to the target. The net result is a nearly uniform
distribution of rays over the target surfaces as shown.
The electrostatic lens of the present invention functions in an
analogous way. With reference now to FIG. 3, a cylindrical anode 23
having a potential V and an inner double conical grid 20 at ground
potential are shown. Within r.sub.o, a zero (field free) region is
formed while an outer radial field simultaneously exists between
the anode 23 and the wire grid 20 (r>r.sub.o). Like the optical
ring lens of FIGS. 1 and 2, the electrostatic lens of FIG. 3 has
two concentric lens regions; in this case they are separated by a
grid conductor that must be highly transparent to the incoming ion
beam. The dimensions r.sub.o, 3r.sub.o, and L shown in FIG. 3 are
discussed in more detail in a theoretical paper of mine published
in Nuclear Instruments and Methods, Vol. 127, pp. 163-171, 1975,
which is incorporated by reference thereto.
One embodiment of a lens assembly meeting desired focusing and
transparency requirements of the present invention and discussed in
the above-mentioned paper is shown in FIG. 4. It was built for
r.sub.o = 0.33 cm and L = 8 cm. Two grid support rings 15 and 16
are separated by two spacer rods 17, and a second pair of spacer
rods 19 separate a spring support ring 18 and the ring 16. A double
conical grid 20 represents a compromise between conflicting demands
of uniformity of the radial field, transparency to the incident ion
beam, and mechanical stability and simplicity. The grid 20 has
twenty-four equally spaced 0.005-cm diameter wires (either
molybdenum or tungsten) attached to the support ring 15, threaded
through a grid ring 22 and through the support ring 16 where they
are fastened to a set of springs 21 that are, in turn, fastened to
the spring support ring 18. The wires 20 are under tension so as to
remain stable when heated by ion bombardment.
An insulated anode 23 surrounding the grid 20 is split lengthwise
for installation after the grid is constructed, and is connected
electrically to receive a high voltage (V) from a source, not
shown. It can be seen from FIGS. 3 and 4 that the interior of the
anode 23 is flared outwardly at each end thereof to make the
average field experienced by the ions at any given radius outside
of r.sub.o nearly independent of that radius. The other components
of FIG. 4, including the grid 20, are at ground potential. The
entire lens assembly is mounted in a vacuum housing, not shown,
having an electrical feedthrough for the anode voltage and means
for coupling a source of high energy ions for entrance through the
vacuum housing and then through the lens of FIG. 4.
FIG. 5 shows a typical beam path from a 5.5-MV electrostatic ion
accelerator with the lens of FIG. 4 installed in the beam line 1
thereof. Components to the left of the lens in FIG. 5 are all
associated with the incident beam into the lens, the lens is near
the middle, and a rectangular aperture and beam profile monitor are
near the target to the right.
Components associated with the incident beam are the analyzer and
slits, the steering plates, and the quadrupole singlet. The
analyzer is a 90.degree. double-focusing magnet which, together
with the slits, serves to analyze and control the beam from an
accelerator, not shown. Since the unanalyzed beam has small
divergence, the crossover formed near the focal points of the
magnet is small, about 1 mm diameter. This crossover is an ion
object which is essentially fixed in space, so the object-target
axis is well-defined. It is important that the other components not
disturb this axis. Therefore, the electrostatic steering plates,
which are needed to correct for angular deviations of the beam from
the axis, are placed near the object to avoid formation of an
off-axis virtual object. Also, the quadrupole for symmetrization of
the beam 1, when needed, is placed near the object to minimize the
axial extent of the virtual object for the ring lens.
The electrostatic lens of the present invention when installed in
the beam line of the 5.5-MV electrostatic accelerator has been
operated without failure for a few hundred hours with incident 1 to
5 particle-.mu.amp beams of 4-MeV doubly-charged .sup.58 Ni ions,
and it has been determined that the lens is a reliable device that
transforms a gaussian or similarly divergent and symmetric ion beam
into a nearly uniform distribution on a target. For example, in one
test, 30% of a 4-MeV .sup.58 Ni ion beam was focused onto a 7 mm
.times. 10 mm rectangular target array with a uniformity of about
.+-. 15%. This intensity represents an increase of 2 to 3 times
that which was previously achievable.
This invention has been described by way of illustration rather
than by limitation and it should be apparent that it is equally
applicable in fields other than those described.
* * * * *